Small Leucine Rich Proteoglycans (SLRPs) are an expanding family of proteoglycans found in the extracellular matrix that contain multiple tandem repeats of a motif rich in leucine. The most abundant SLRP in bone is biglycan (BGN) and is, currently, the major focus of our studies. To determine the functions of BGN in vivo transgenic mice were created that were deficient in the production of the protein (knockout/KO). These mice acquired diminished bone mass that was progressive with age. Double tetracyline-calcein labeling revealed that the BGN deficient mice were defective in their capacity to form bone. To determine the cellular basis for the skeletal defect, bone cells were isolated from the marrow of normal and mutant mice at 6, 12 and 24 weeks of age and the number of bone cell precursors (CFUf) quantitated. A dramatic age dependent decrease in colony forming units was observed in the BGN KO animals compared to normal littermates. Northern analysis showed that type I collagen mRNA was also diminished indicating that both the quantity and quality of the bone forming cells was affected in the BGN KO mice. Electron Microscopy (EM) analysis showed that bones isolated from BGN KO mice had collagen fibrils that were more "irregular" in shape. Such structural defects may be causative factors in the compromised bone mineralization and biomechanical strength previously observed in the mutant mouse lines. Western analysis of the bone showed that BGN KO mice contain substantial amounts of the highly related SLRP decorin (DCN). To determine if DCN additionally contributes to bone tissue integrity we generated mice deficient in both BGN and DCN. The "double KO" mice had substantial decreases in both trabecular and cortical bone mass compared to WT, BGN or DCN KO animals implicating a compensatory role of DCN in the absence of BGN. A major challenge will be to understand how gene expression patterns are altered in the SLRP KO mice and, further, how the responses of growth factor and hormones are affected. These studies will be necessary to untangle the complex network of SLRP function in bone at the tissue, cell and molecular levels.